let str: string | undefined = undefined
console.log("str:", str);

function fn(data?: string) {
    // data!.toString()
    if (data) {
        console.log(data.toUpperCase())
    }
}

fn()

// any,unknown,undefined 可以接收 undefined
// any, unknown,null 可以接收 null
let data1:undefined = undefined
let data2:any = undefined
let data3:unknown = undefined


export { }
